1. Technical Field
The present invention relates to the field of optical devices.
2. Related Art
In recent years, some biometric devices and image scanners have a light-emitting portion and a light-receiving portion that are disposed on the same side with respect to a subject (e.g., a part of a living body, an original, and the like) placed on a reading area and read an image of the subject by irradiating the subject with light from the light-emitting portion and receiving the reflected light with the light-receiving portion. For example, JP-A-2009-172263 discloses a biological information acquisition device that captures a vein image of the finger by irradiating the finger with light (near-infrared light) emitted from a light source using a plurality of reflecting surfaces formed on a rear surface of a light guide plate, and receiving light reflected from the finger with a light receiving element having a plurality of pixels.
However, there is a problem with the biological information acquisition device disclosed in JP-A-2009-172263 in that it is difficult to capture a sharp image. That is to say, the emitted light (noise light) that may possibly have a larger light quantity than the reflected light (signal light), from which information should be read, is introduced into the light receiving element, resulting in a decrease in the ratio of the signal light to the noise light (S/N ratio). The reasons for this are that the light guide plate that guides the emitted light from the light source, which has a larger light quantity than the reflected light, is present on optical paths (on straight lines extending from the respective pixels in a vertical direction) of the reflected light incident on the pixels, and that a part of the emitted light from the light source is transmitted through a low refractive index layer and a reflective layer (semi-reflective layer) that are formed on the rear surface side of the light guide plate and leaks to the side of the light receiving element. Consequently, there is a problem in that the light receiving element cannot accurately receive the reflected light from the finger.